This invention relates to a system for etching or developing photoresist and, in particular, to an apparatus and method for replenishing developer in such a way as to maintain the composition of the developer as constant as possible.
A printed circuit board consists of an insulating substrate, such as phenolic or fiberglass, and at least one adherent, conductive layer. The conductive layer completely covers a major surface of the substrate and, at this stage, does not define a circuit. A circuit is defined by coating the conductive layer with photoresist, patterning the photoresist, and transferring the pattern to the conductive layer.
Photoresist is a material which chemically changes in response to actinic radiation, typically incident light. The photoresist is exposed to an image of a circuit pattern, thereby changing the solubility of selected portions of the photoresist layer in accordance with the circuit pattern. Boards having an exposed photoresist layer are mounted on a conveyer and dipped or sprayed with solvent or developer. Typically, a spray booth is used having an array of spray nozzles fed by a pump connected to a sump in the bottom of the booth which collects the developer running off the boards.
The photochemical change in the photoresist is not absolute, i.e. the photoresist does not change from completely soluble to completely insoluble. Therefore, it is important to develop the photoresist only long enough to remove the photoresist from those areas where it is intended to be removed. In the remaining areas, the photoresist may become thinner but is not completely removed. The point of complete development is referred to as the breakpoint and should occur prior to the board exiting the spray zone in the booth, typically sixty percent through the zone. Under-developing can cause short circuits in finished printed circuit boards. Overdeveloping can cause changes in the geometry of the lines in the photoresist, e.g. a line will become narrower as well as thinner, possibly causing an open circuit. For these reasons, a correct breakpoint is critical for printed circuit boards having patterns of closely spaced, fine lines.
As used herein, "develop" refers to the process for forming a pattern in a layer irrespective of how the process is accomplished. That is, the pattern can be formed by dissolving portions of the layer or by etching, i.e. chemically reacting with portions of the layer to form soluble or volatile compounds.
The photoresist remaining on a board after development is used as a mask for the underlying conductive layer, transferring the circuit pattern from the photoresist to the conductive layer. After the conductive layer is patterned and the remaining photoresist is removed, a board is typically coated with a solder mask layer, giving the board the greenish appearance seen on the "foil" or circuit side of the board.
The breakpoint is determined by the reaction rate of the developer which, in turn, depends on a number of factors including the temperature and concentration of the developer. As can be imagined, using a developer until it is exhausted causes a continuously changing breakpoint and, consequently, can cause significant differences between supposedly identical printed circuit boards. In a typical batch replenishment operation, development time can increase by as much as fifty percent as the developer reaches exhaustion. Batch replenishment, in which the sump is completely drained and re-filled with fresh developer, causes significant down time and quality control problems and a suitable alternative has been sought for a long time.
Continuously replenishing a developer sump, in which a small fraction of the capacity of the sump is pumped into the sump for each board processed, requires a large reservoir for the replenishing solution and may not solve the problem. The consumption of developer depends upon the amount of photoresist being removed, which is determined by the circuit pattern and the size of each board. This is referred to as the "loading" on the developer. Since the loading varies unless an entire shift is devoted to producing the same circuit board, continuously replenishing the developer will not produce uniform results.
For environmental and safety reasons, an inorganic, water soluble developer is used, e.g. potassium carbonate, and the reaction with the photoresist produces a bicarbonate and dissolved photoresist resin. The concentrations of carbonate, bicarbonate, and resin determine the pH of the developer. It is known in the art to monitor pH as an indication of the condition of the developer since an increasing concentration of dissolved resin decreases the pH of the developer.
A problem with monitoring pH for automatic replenishment is that the pH electrode quickly becomes coated with scale from tap water and dissolved resin. A commercially available system avoids contamination of the pH electrode by periodically flushing the electrode with an acid solution. The same system avoids using a large reservoir for the replenishing solution by mixing developer as needed from concentrated developer. Mixing is controlled by a solenoid valve on a water line and by a metering pump for the concentrated developer. While this system is a significant improvement over previous systems, problems remain. The acid bath shortens the life of the pH electrode and the mixing of replenishment solution is not sufficiently accurate to maintain constant breakpoint. Water mains vary considerably in pressure and temperature and metering pumps must be calibrated frequently to maintain constant flow.
In view of the foregoing, it is therefore an object of the invention to provide an improved method and apparatus for automatically replenishing developer.
Another object of the invention is to provide a method and apparatus for maintaining constant breakpoint in developer.
A further object of the invention is to provide an improved method and apparatus for mixing solutions accurately.
Another object of the invention is to provide a more consistent reaction rate by matching the temperature of the replenishment solution to the temperature of the developer.
A further object of the invention is to provide a compact replenishment system that is accurate and does not require expensive, precise components.